The invention concerns an implant for implanting into human tissue of the type including a hermetically sealed, metallic housing for enclosing electronic means inside the housing for a high-frequency, telemetric information transfer between a programming device and the implant, the implant including a head for connecting a stimulation line. Such an implant is known from German patent document DE 27 20 011 C2.
The contact between the electronic components and the salt-containing and therefore conductive body fluid must be avoided for implants that can be implanted into human tissue, e.g. a pacemaker, as this will otherwise lead to signs of corrosion and electrical malfunctions of the pacemaker. A hermetically sealed metal housing is provided for this, which protects the sensitive electronic componentry of the pacemaker to a sufficient degree. However, such metal capsules have the disadvantage of leading to a more or less strong screening of magnetic and electromagnetic alternating fields during the pacemaker operation, which fields are used for an energy or signal input or discharge into or from the pacemaker. On the one hand, this energy loss, which is essentially caused by eddy currents, makes itself felt by a heating up of the housing frequently perceived as physiologically troublesome by the patient while, on the other hand, it requires a larger amount of energy for a secure signal transmission over a specific distance. This constitutes a high stress, particularly for the energy storage device present in the pacemaker, which has the disadvantage of causing a shortening of the discharge time for the energy storage device used. The eddy current losses increase with an increase in the frequency and limit the range of the telemetric data transmission in addition to the data throughput rate.
The conductivity and thickness of the housing material determine the size of the eddy currents. The thicker the material and the higher its conductivity, the larger the eddy currents caused by telemetric transmissions.
The above-mentioned document DE 27 20 011 C2 discloses a rechargeable pacemaker, having a smooth, polished housing wall composed of a biocompatible material and a uniform thickness in the range of 0.05 mm to 0.13 mm. The energy input for charging up the battery inside the housing is supplied to the pacemaker via an external, magnetic alternating field, wherein the energy losses that lead to a troublesome warming up of the housing caused by eddy currents, must be reduced through specific dimensioning of the measurements. That is the reason why the quotient for the material strength and specific electrical resistance of the metallic material used for the pacemaker housing is selected to be below a predetermined limit value.
Since it is only the material strength of the housing wall that determines this quotient for a selected, biocompatible metallic material, the use of this solution is limited in a disadvantageous way. These limits are basically predetermined by a minimum wall thickness of the housing, which is necessary for the safe operation of the pacemaker, meaning mechanical stability and imperviousness to the body fluid surrounding the pacemaker.